threads_internal: pass the current 'action' to the internal thread system

threads_internal: pass the current 'action' to the internal thread system

I build a ModelAction object to represent the properties of the current action
that are relevant to the model checker. This action should be passed from the
basic action (atomic_load, atomic_store, thread_yield, etc.) into our thread
library and eventually to the model checker.

I don't use these actions yet, but they will be shortly.